The long-term objective of this proposal is to understand the role of caveolin-1 in i) signaling/cell transformation and ii) tumorigenesis. Tumorigenesis is a multi-step process that involves a series of genetic changes or "multiple hits" (i.e., inactivation of a tumor suppressor gene(s) and activation of a proto-oncogene(s), leading to alterations in signal transduction and cell transformation. Several lines of in vitro evidence suggest that the Cav-1 gene may function as a novel tumor suppressor. Genetic validation of this hypothesis at the cellular and whole-organismal level has been prevented by the lack of a Cav-1 null mouse model. Here, we will test the hypothesis that Cav-1 functions as a tumor suppressor gene in vivo, using Cav-1 (-/-) null mice as a model. We will assess whether loss of Cav-1--in combination with an oncogenic stimulus--leads to increases in cell transformation and tumor formation. For this purpose, we will use three distinct oncogenic stimuli: a chemical carcinogen (DMBA; SA 1); loss of a tumor suppressor (INK4a; SA 2); and, addition of an activated oncogene (PyMT; SA 3). We will also employ cultured cells (keratinocytes, fibroblasts, and mammary epithelia) derived from Cav-1 null animals. The three Specific Aims (SA) of the project are: 1) To determine if loss of Cav-1 sensitizes mice to carcinogen-induced epidermal hyperplasia and skin tumor formation. Our preliminary results show that Cav-1 null mice are dramatically more susceptible to carcinogen-induced skin tumorigenesis, using DMBA. We will prepare primary keratinocytes from these animals and study their proliferative capacity, alterations in signal transduction, and differentiation state; 2) To examine the co-operativity of Cav-1 with other tumor suppressors, such as the INK4a locus. We will generate mice that are doubly null for Cav-1 and INK4a, and characterize their phenotype. We will prepare mouse embryo fibroblasts (MEFs) from these mice and assess their capacity for proliferation, anchorage-independent growth, and tumor formation in nude mice. Our preliminary results indicate that Cav-1 and INK4a show strong co-operativity; and 3) To explore the role of Cav-1 in mammary tumor formation, using cell-specific ablation of Cav-1. Our preliminary results indicate that whole-body ablation of Cav-1 accelerates the development of dysplastic lesions and mammary tumors in MMTV-PyMT mice. However, this tumor suppressor effect is diminished with time, due to a concomitant angiogenesis defect. Therefore, cell-type specific ablation of Cav-l, using the Cre/loxP system, will allow us to separately evaluate the contribution of loss of Cav-1 in mammary epithelia vs. endothelia vs. fatty stromal cells (mammary adipocytes), in the context of mammary tumorigenesis.